An ink jet recording method is a printing method of ejecting a droplet of an ink composition to attach it onto a recording medium such as paper, thereby performing printing. This ink jet recording method is characterized by that images having high resolution and high quality can be printed at high speed. The ink composition used in the ink jet recording method is generally one mainly composed of an aqueous solvent and containing a coloring material component and a wetting agent such as glycerin for the purpose of preventing clogging.
Further, when printing is performed on paper or cloth into which the aqueous ink composition is difficult to penetrate, or a material such as metal or plastic into which the aqueous ink composition does not penetrate, for example, a recording medium such as a plate or a film made of a resin such as a phenol resin, a melamine resin, a vinyl chloride resin, an acrylic resin or a polycarbonate resin, the ink composition is required to contain a component which can allow the coloring material to be stably firmly fixed to the recording medium. In particular, when printing is performed on a print wiring substrate or the like, quick drying properties and chemical resistance are demanded for the ink composition.
To such a demand, an ink composition comprising a component polymerizable by ultraviolet irradiation has hitherto been proposed (for example, see patent document 1). Further, an ultraviolet curing type ink composition comprising a coloring material, an ultraviolet curing agent, photopolymerization initiator and the like has been proposed (for example, see patent document 2). According these ink compositions and ink jet recording methods, it is said that blurring of the ink composition on the recording medium can be prevented to improve image quality.
In the ink jet recording method using the ink composition comprising the component polymerizable by ultraviolet irradiation as described above, an ultraviolet lay is irradiated after the ink composition has been adhered to the recording medium. Thereupon, the photopolymerization initiator in the ink composition forms radicals and the like, thereby initiating polymerization of an oligomer and a monomer, followed by curing. Accordingly, the coloring material in the ink composition is firmly fixed onto the recording medium. This firm fixing is considered to be able to realize printing which is high in film strength, solvent resistance and color density and has decreased blurring and unevenness.
It has been known that monomers used in the conventional ultraviolet curing inks generally have the characteristics that one having high curability is high in the primary irritation index (PII) and one having low PII is also low in curability. Accordingly, in the conventional ultraviolet curing ink compositions, materials having a PII of 5 or more are used as raw materials in many cases in order to enhance the rate of polymerization, and attention is required for handling thereof. An equipment burden such as the use of a local exhaust system is necessary in use, and this has been unfavorable.
Further, the rate of polymerization by ultraviolet curing generally tends to increase as the molecular weight of a monomer used decreases.
From the relationship between the high and low of the above-mentioned PII and the high and low of curability (rate) and the relationship between the high and low of the rate of ultraviolet curing polymerization and the high and low of the molecular weight of the monomer used (in the case of similar structures), the lower the molecular weight of the monomer used, the higher the rate of polymerization, but the PII is also high. The higher the molecular weight of the monomer used, the lower the PII, but the rate of polymerization is also low, and further, the viscosity also becomes high (for example, in order to decrease the PII, a methacrylate monomer is more effective than an acrylate monomer, but the rate of polymerization decrease, and further, the viscosity becomes high).
Furthermore, it is also advantageous to use a multifunctional monomer as a measure for improving the low PII and the rate of polymerization, but the viscosity becomes high. In order to decrease the viscosity of the high-viscosity ink composition, an organic solvent (VOC) which is high in volatility (evaporates at ordinary temperature) is forced to be used in large amounts. This has not been favorable also from the viewpoint of the environment.
Patent Document 1: JP-A-3-216379
Patent Document 2: U.S. Pat. No. 5,623,001
As described above, various attempts have been made to the demands for decreasing the PII, improving the rate of polymerization and decreasing the viscosity. However, development of an ink composition which meets all “the high rate of polymerization, the low PII value and the low viscosity” has not been achieved.